One known navigation system is the GPS system (Global Positioning System) which presently comprises more than 20 satellites, of which, usually, a half of them are simultaneously within the sight of a receiver. These satellites transmit e.g. Ephemeris data of the satellite, as well as data on the time of the satellite. A receiver used in positioning normally deduces its position by calculating the propagation time of a signal received simultaneously from several satellites belonging to the positioning system to the receiver and calculates the time of transmission (ToT) of the signals. For the positioning, the receiver must typically receive the signal of at least four satellites within sight to compute the position. The other already launched navigation system is the Russian-based GLONASS.
In the future, there will also exist other satellite based navigation systems than GPS and GLONASS. In the Europe the Galileo system is under construction and will be launched within a few years. Space Based Augmentation Systems SBAS (WAAS, EGNOS, GAGAN) are also being ramped up. Local Area Augmentation Systems LAAS, which uses fixed navigation stations on the ground, are becoming more common. Strictly speaking, the Local Area Augmentation Systems are not actually satellite based navigation systems although the navigation stations are called as “pseudo satellites” or “pseudolites”. The navigation principles applicable with the satellite based systems are also applicable with the Local Area Augmentation Systems. Pseudolite signals can be received with a standard GNSS receiver. Moreover, Japanese are developing their own GPS complementing system called Quasi-Zenith Satellite System QZSS.
The satellite based navigation systems, including systems using pseudo satellites, can collectively be called as Global Navigation Satellite Systems (GNSS). In the future there will probably be positioning receivers which can perform positioning operations using, either simultaneously or alternatively, more than one navigation system. Such hybrid receivers can switch from a first system to a second system if e.g. signal strengths of the first system fall below a certain limit, or if there are not enough visible satellites of the first system, or if the constellation of the visible satellites of the first system is not appropriate for positioning. Simultaneous use of different system comes into question in challenging conditions, such as urban areas, where there is limited number of satellites in view. In such cases, navigation based on only one system is practically impossible due to the low availability of signals. However, hybrid use of different navigation systems enables navigation in these difficult signal conditions.
Each satellite of the GPS system transmits a ranging signal at a carrier frequency of 1575.42 MHz called L1. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, the modulation of these signals is performed with at least one pseudo random sequence. This pseudo random sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible in the receiver to distinguish between the signals transmitted from different satellites, although the carrier frequencies used in the transmission are substantially the same. Doppler effect results in a small (±5 kHz) change in the carrier frequency depending upon the constellation geometry. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudo sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with a polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to produce different C/A codes with an identical code generator. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the code epoch is 1 ms. The L1 carrier signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the health of the satellite, its orbit, clock behaviour, etc.
In the GPS system, satellites transmit navigation messages including Ephemeris data and time data, which are used in the positioning receiver to determine the position of the satellite at a given instant. These Ephemeris data and time data are transmitted in frames which are further divided into subframes. FIG. 6 shows an example of such a frame structure FR. In the GPS system, each frame comprises 1500 bits which are divided into five subframes of 300 bits each. Since the transmission of one bit takes 20 ms, the transmission of each subframe thus takes 6 s, and the whole frame is transmitted in 30 seconds. The subframes are numbered from 1 to 5. In each subframe 1, e.g. time data is transmitted, indicating the moment of transmission of the subframe as well as information about the deviation of the satellite clock with respect to the time in the GPS system.
The subframes 2 and 3 are used for the transmission of Ephemeris data. The subframe 4 contains other system information, such as universal time, coordinated (UTC). The subframe 5 is intended for the transmission of almanac data on all the satellites. The entity of these subframes and frames is called a GPS navigation message which comprises 25 frames, or 125 subframes. The length of the navigation message is thus 12 min 30 s.
In the GPS system, time is measured in seconds from the beginning of a week. In the GPS system, the moment of beginning of a week is midnight between a Saturday and a Sunday. Each subframe to be transmitted contains information on the moment of the GPS week when the subframe was transmitted. Thus, the time data indicates the moment of transmission of a certain bit, i.e. in the GPS system, the moment of transmission of the last bit in the subframe. In the satellites, time is measured with high-precision atomic chronometers. In spite of this, the operation of each satellite is controlled in a control centre for the GPS system (not shown), and e.g. a time comparison is performed to detect chronometric errors in the satellites and to transmit this information to the satellite.
During their operation, the satellites monitor the condition of their equipment. The satellites may use for example so-called watchdog operations to detect and report possible faults in the equipment. The errors and malfunctions can be instantaneous or longer lasting. On the basis of the health data, some of the faults can possibly be compensated for, or the information transmitted by a malfunctioning satellite can be totally disregarded. The malfunctioning satellite sets a flag in a satellite health field of a navigation message indicative of a failure of the satellite. It is also possible that a Control Segment of a Satellite Navigation System detects abnormalities in the operation of a satellite or in signals of a satellite. Hence, the Control Segment can also set a non-healthy indication for such a satellite. This indication can also be set when a satellite is tested or during a possible correction operation of the orbit of the satellite.
It is also possible to detect abnormalities in the operation of a satellite by examining signals transmitted by a satellite. For example, an observing station may perform measurements of residuals of a pseudorange and if the residual deviates from a computational residual more than a predetermined threshold, the observing station determines that the satellite is not operating properly. Another option is to compare the accuracy of the ephemeris data transmitted by a satellite to a reference data.
The number of satellites, the orbital parameters of the satellites, the structure of the navigation messages, etc. may be different in different navigation systems. Therefore, the operating parameters of a GPS based positioning receiver may not be applicable in a positioning receiver of another satellite system. On the other hand, at least the design principles of the Galileo has indicated that there will be some similarities between GPS and Galileo in such a way that at least Galileo receiver should be able to utilize GPS satellite signals in positioning.
Positioning devices (or positioning receivers) i.e. devices which have the ability to perform positioning on the basis of signals transmitted in a navigation system can not always receive strong enough signals from the required number of satellites. For example, it may occur that when a three-dimensional positioning should be performed by the device, it can not receive signals from four satellites. This may happen indoors, in urban environments, etc. Methods and systems have been developed for communications networks to enable position in adverse signal conditions. If the communications network only provides navigation model assistance to the receiver, the requirement for a minimum of three signals in two-dimensional positioning or four signals in three-dimensional positioning does not diminish. However, if the network provides, for instance, barometric assistance, which can be used for altitude determination, then three satellites is enough for three-dimensional positioning assuming the positioning receiver has access to barometric measurements (e.g. from an integrated barometer). These so called assisted navigation systems utilise other communication systems to transmit information relating to satellites to the positioning devices. Respectively, such positioning devices which have the ability to receive and utilize the assistance data can be called as assisted GNSS receivers, or more generally, assisted positioning devices.
Currently, only assistance data relating to GPS satellites can be provided to assisted GNSS receivers in CDMA (Code Division Multiple Access), GSM (Global System for Mobile communications) and W-CDMA (Wideband Code Division Multiple Access) networks. This assistance data format closely follows the GPS navigation model specified in the GPS-ICD-200 SIS (SIS, Signal-In-Space) specification. This navigation model includes a clock model and an orbit model. To be more precise, the clock model is used to relate the satellite time to the system time, in this case the GPS time. The orbit model is used to calculate the satellite position at a given instant. Both data are essential in satellite navigation.
The availability of the assistance data can greatly affect the positioning receiver performance. In the GPS system, it takes at least 18 seconds (the length of the first three subframes) in good signal conditions for a GPS receiver to extract a copy of the navigation message from the signal broadcasted by a GPS satellite. Therefore, if no valid copy (e.g. from a previous session) of a navigation model is available, it takes at least 18 second before the GPS satellite can be used in position calculation. Now, in AGPS receivers (Assisted GPS) a cellular network such as GSM or UMTS (Universal Mobile Telecommunications System) sends to the receiver a copy of the navigation message and, hence, the receiver does not need to extract the data from the satellite broadcast, but can obtain it directly from the cellular network. The time to first fix (TTFF) can be reduced to less than 18 seconds. This reduction in the time to first fix may be crucial in, for instance, when positioning an emergency call. This also improves user experience in various use cases, for example when the user is requesting information of services available nearby the user's current location. These kind of Location Based Services (LBS) utilize in the request the determined location of the user. Therefore, delays in the determination of the location can delay the response(s) from the LBS to the user.
Moreover, in adverse signal conditions the utilization of the assisted data may be the only option for navigation. This is because a drop in the signal power level may make it impossible for the GNSS receiver to obtain a copy of the navigation message. However, when the navigation data is provided to the receiver from an external source (such as a cellular network), navigation is enabled again. This feature can be important in indoor conditions as well as in urban areas, where signal levels may significantly vary due to buildings and other obstacles, which attenuate satellite signals.
When a mobile terminal having an assisted positioning receiver requests for assistance data, the network sends the mobile terminal one navigation model for each satellite in the view of the assisted positioning receiver. The format in which the assistance data is sent is specified in various standards. Control Plane solutions include RRLP (Radio Resource Location Services Protocol) in GSM, RRC (Radio Resource Control) in W-CDMA and IS-801.1/IS-801.A in CDMA. Broadcast assistance data information elements are defined in the standard TS 44.035 for GSM. Finally, there are User Plane solutions OMA SUPL 1.0 and various proprietary solutions for CDMA networks. The common factor for all these solutions is that they only support GPS. However, due to the ramp up of Galileo, all the standards shall be modified in the near future in order to achieve Galileo compatibility.
The international patent application publication WO 02/67462 discloses GPS assistance data messages in cellular communications networks and methods for transmitting GPS assistance data in cellular networks.
The navigation systems index the satellites to express the satellite the information relates to. This is called as satellite indexing. The satellite index is used to identify the navigation model with a specific satellite. Every navigation system has its own indexing method.
GPS indexes satellites (SV, Space Vehicle) based on PRN (PseudoRandom Noise) numbers. The PRN number can be identified with the CDMA spread code used by the satellites.
Galileo uses a 7-bit field (1-128) to identify the satellite. The number can be identified with the PRN code used by the satellite.
GLONASS uses a 5-bit field to characterize satellites. The number can be identified with the satellite position in the orbital planes (this position is called a “slot”). Moreover, in contrast to other systems, GLONASS uses FDMA (Frequency Division Multiple Access) to spread satellite broadcasts in spectrum. It is noted here that there is also a CDMA spread code in use in the GLONASS. There is, therefore, a table that maps the satellite slot number to the broadcast frequency. This map must be included in any assistance data format.
SBAS systems use PRN numbers similar to GPS, but they have an offset of 120. Therefore, the first satellite of the SBAS system has a satellite number of 120.
Since QZSS SIS ICD is not public yet, there is no detailed information on the satellite indexing in the system. However, since the system is a GPS augmentation, the GPS compatible format should at high probability be compatible with QZSS as well.
Pseudolites (LAAS) are the most problematic in the indexing sense. There is no standard defined for indexing pseudolites currently.
However, the indexing should at least loosely follow the GPS-type indexing, since they use GPS-type PRNs. Therefore, by ensuring that the range of satellite indices is sufficient, it should be possible to describe LAAS transmitters with GPS-type satellite indexing.
In addition to these requirements (indexing, clock model and orbit model), the navigation model must include information on model reference time (tREFERENCE in the clock model, similar time stamp is required for the orbit model), model validity period, issue of data (in order to be able differentiate between model data sets), SV health (indicates whether navigation data from the SV is usable or not).
The current satellite health field requires modification, since in the future GPS (and other systems) do not transmit only one signal, but various signals at different frequencies. Then, it is possible that one of these signals is unusable, but others are fine. Then, the satellite health must be able to indicate this mode of malfunction. Current solution in GPS is only able to express malfunction is some signal (without specifying which one). The problem was previously solved only by saying that the whole satellite is broken and not just some specific signal.